Proline-directed phosphorylation (pSer/Thr-Pro) is a major signaling mechanism in the cell. Although these phosphorylation events had been long proposed to regulate protein function by inducing conformational changes, little was known about the nature, significance and regulation of such conformational changes until recently. Our identification of a unique enzyme, Pin1 that isomerizes specific pSer/Thr-Pro bonds in certain proteins suggested a new signaling mechanism, whereby Pin1 catalytically regulates the conformation of its substrates after phosphorylation to control protein function. Recent studies have shown that such Pin1- catalyzed conformational changes can have profound effects on many key proteins in diverse cellular processes. Importantly, Pin1 deregulation contributes to some pathological conditions, notably cancer and Alzheimer's disease. In breast cancer, we have shown that Pin1 is an E2F target gene that is critical for the regulation of multiple upstream oncogenic signal pathways, and for the coordination of some downstream cell cycle events such as centrosome duplication. Moreover, Pin1 knockout prevents certain oncogenes from inducing breast cancer, whereas Pin1 overexpression causes centrosome amplification, aneuploidy and breast cancer. Thus, Pin1 plays a critical role in oncogenesis and is a potential anticancer target. However, the major challenges include whether and how to control Pin1 function and what are other novel functions for Pin1 in growth regulation. Our published and preliminary results suggest that Pin1 is subject to multiple post- translational modifications, including inhibitory phosphorylation, acetylation and methylation, and that Pin1 might be phosphorylated by a kinase that is a known tumor suppressor. Therefore, Aim 1 will be to determine the significance of Pin1 phosphorylation and other novel post-translational modifications in regulating Pin1 function during cell proliferation and transformation, and to identify enzymes responsible for Pin1 modifications. Moreover, our preliminary studies uncovered that Pin1 bound to and increased turnover of Pin2/TRF1, which was also identified in the same genetic screen for Pin1 and is important for mitotic regulation. Given that TRF1 is a telomeric protein negatively regulating telomere elongation, Aim 2 will be to elucidate the novel role and mechanisms of Pin1 in regulating TRF1 function in telomere maintenance and mitotic progression in normal and cancer cells. These studies should help understand the role of post- phosphorylation regulation in oncogenesis and might have novel therapeutic implications. In lay language, we have identified a new enzyme called Pin1 critical for cancer development and will continue to investigate the function and regulation of Pin1 during cancer development and hope to identify new drug targets.